Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

642
FXUS63 KLOT 161937
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
237 PM CDT Tue Jun 16 2026

.KEY MESSAGES...

- Threat for isolated to widely scattered thunderstorms near and
  east of I-55 this afternoon, with a level 1 to 2 of 5 severe
  risk, including northwest Indiana.

- A dangerous severe weather outbreak, including the potential
  for long tracked, strong-violent tornadoes, is possible
  Wednesday, especially south of I-80.

- Periods of torrential rainfall Wednesday could result in areas
  of flash flooding (some locally significant) and potentially
  some rapid onset river flooding.

&&

.UPDATE...
Issued at 1036 AM CDT Tue Jun 16 2026

Thinking remains valid for this afternoon`s threat.

Regarding this afternoon`s threat for strong to severe
thunderstorms, SPC`s updated day 1 outlook highlighting a level
2 of 5 in northwest Indiana and points east appears reasonable.
The features of note for the setup this afternoon are: a cold
front approaching the MS River as of this writing; and an
unusually cold (for June) mid-upper low pivoting towards the
western Lakes from the upper MS Valley. 500 mb temps of -19C
sampled by the 12z MPX RAOB are below the 10th percentile for
mid June and not far off daily mins for that site.

Modest surface convergence along the cold front and robust large
scale forcing are expected to eventually yield widely scattered
storms just ahead of the front. The big question mark is when
and where will development occur? Comparing dew point observations
to modeled dew points from the 12z suite of higher resolution
guidance at this time, the models with lower dew points are
verifying closer to reality. With the likelihood of veering
winds in the frontal trough, lower dew points, and less time
until cold frontal approach may limit destabilization enough to
preclude renewed convective initiation until near or over Lake
Michigan and northwest Indiana. It`s still close enough to
warrant holding onto chance PoPs in and near Chicago, with
higher PoPs east of I-57 or so between about 2-5pm CDT. Even
with a (likely) closer to reality dew point depiction, the 12z
WRF-ARW does depict CI right near/over the city around 1-2pm
before quickly shifting east. On the other hand, the alternative
portrayed by the HRRR of storms just developing east and
southeast of our CWA is also plausible.

Potential severe hazards wise (if any severe storms occur in the
CWA), the main threats look to be damaging hail and winds, in
the presence of 40-50 kt of deep layer vertical shear and cold
temps aloft/steep lower to mid-level lapse rates, both
supportive of low-topped supercells. SPC also highlighted a
corridor of tornado potential, though it`s unclear if LCLs will
be prohibitively high. We`ll certainly be keeping a close eye on
observational, satellite, and guidance trends over the next few
to several hours.

Castro

&&

.DISCUSSION...
Issued at 237 PM CDT Tue Jun 16 2026

Tonight:

This is the calm before the proverbial storm as winds quickly
diminish with sunset. Expect temporary mostly clear skies before
mid and high level cloud cover overspreads the region in
advance of Wednesday`s powerful storm system. While the
instability axis will remain off to our west through daybreak,
some isolated to scattered showers may move across portions of
northern and central Illinois between about 4 AM and 7 AM.

Wednesday:

Synoptically, the ingredients appear likely to come together for
a potentially dangerous severe weather outbreak somewhere over
the broader region (favoring I-80 and south). This includes
potentially long tracked intense tornadoes and destructive
straight line wind gusts (and line embedded tornadoes).

Meteorological Setup:

An early-spring like (vs. mid June in actuality) mid and upper
level pattern in place across northern North America features
exceptionally strong west-northwesterly flow aloft around an
unseasonably deep closed low southwest of Hudson Bay. A potent
neutrally tilted short-wave trough and associated intense upper
level jet streak will eject southeastward and result in near
record strong surface cyclogenesis (upper 980s to low 990s mb
surface low) over Minnesota into Wisconsin by early Wednesday
afternoon.

Impressively strong warm and moist advection atop a lingering
stable boundary layer and a rapid increase in large scale
forcing will quickly join forces Wednesday morning. The likely
result will be an area of convection propagating east-southeast
across the area through early afternoon (more on the potential
severe threat with this initial round to follow).

The effective warm front will likely be suppressed south to an
extent in the wake of the morning/early afternoon convection.
However, intense mass response and development of a secondary
surface low will likely result in the sharpening warm front
shifting north during the afternoon. Exactly how far north the
front gets is the million dollar question, so to speak, for the
looming more concerning mid afternoon through early evening
severe threat (discussed below). Regardless of the exact
position of the front, much of if not the entire area is fair
game for waves of intense convection. The system cold front will
sweep across the area between about 7pm and 9pm, quickly
bringing the severe weather threat to an end.

Severe Threat Details:

Round 1 (~8 AM to 1 PM CDT): Initial development of strong to
severe storms over eastern Iowa tomorrow morning should
translate into portions of northern and central Illinois in
tandem with a 60+ kt southwesterly low-level jet by late morning
into early afternoon. Strong effective shear and up to 1-2k
J/kg of MUCAPE could in turn support elevated supercells and a
corresponding threat for damaging to locally destructive winds
and hail. The pattern with this initial round of convection will
be one to watch for gravity wave associated convection, which
would potentially up the ante for the damaging wind threat,
despite little/no surface-based instability.

Our thinking hasn`t changed much with this initial round, with
respect to western and southern/southwestern CWA locales being
most favored for severe weather from "round 1". This certainly
doesn`t preclude the threat extending farther north. There still
could be isolated strong storms with heavy downpours and hails
extending into the Rockford and Chicago metros even if the more
intense convection passes over our southwestern 1/2 (or just
south of that). Finally, we would be remiss to not mention the
possibility of strong wake low winds on the back side of the
initial convection.

Round 2 (2 PM to 9 PM CDT): Presently unknown and unknowable
mesoscale details can be expected to play a notable role in
modulating the locations/corridors of the highest severe threat,
and possibly the magnitude as well (at least within our CWA).
From our assessment, SPC`s depicted level 4 of 5 threat
(moderate risk) area on the updated day 2 outlook continues to
best highlight the most likely areas to have a dangerous severe
weather outbreak. This is tied to where an overlap of incredibly
strong low level (40-50+ kt) and deep layer (70+ kt) shear and
moderately strong instability is most favored to occur.

As already alluded to earlier, the million dollar question is
how far north the sharp warm front is able to push in the wake
of the initial MCS. Lingering cold pool influences are typically
not handled all that well by high-res guidance. In addition, and
on the other hand, we`ll quite literally be in uncharted
territory regarding the expected mass response (recovery back
northward of a volatile parameter space) given the extreme
kinematics at play. The range of depictions in the 12z guidance
suite suggests the reasonable error bar to be between roughly
the US-24 corridor to as far north as near or even just north of
I-80. Thus, a key takeaway here is to absolutely not let one`s
guard down, lest the warm sector push farther north than expected.
Anywhere from near/south of I-88 down to the aforementioned
US-24 corridor is fair game, albeit generally favoring south of
I-80 within the level 4 of 5 threat area.

To the south of the sharp warm frontal boundary (near/south of
the path of the secondary surface low), the progged environment
is akin to those found in winter and early spring severe
weather outbreaks in the southeast US. Discrete or semi-discrete
supercells in such an environment, characterized by 500+ m2/s2
of effective inflow layer SRH and extremely long and looped
hodographs, present a ceiling of long tracked strong to violent
tornadoes. However, with that said, it`s unclear if the storm
mode within our area will lend itself to long tracked and/or
cyclic mesocyclones and tornadoes, or be on the messier/mixed
mode side due to the extremely strong forcing.

In addition, depending on how the boundary layer evolves on the
mesoscale in the wake of the initial MCS, it`s not uncommon to
have a south to north gradient from exceptionally favorable
low-level lapse rates and instability for tornadoes to less
favorable but still sufficient ingredients right near the
northern warm front terminus. Should such a scenario unfold,
this may be a path to modulate/limit the event ceiling tornado
intensity wise, but this is absolutely not something to bank on
in the highest threat areas.

Along with the supercell tornado threat in the system warm
sector, a fast moving line of severe thunderstorms (potential
derecho) may materialize just ahead of the cold front. As we saw
last Thursday, widespread potentially significant wind damage
as well as (potentially strong) line embedded tornadoes are
common in the most intense QLCSs. Considering that effective
shear will be more extreme than Thursday, the above scenario is
certainly on the table for parts of the area. Finally, north of
the sharpening warm front, intense warm and moist advection will
quickly replenish MUCAPE reservoir and could yield scattered
elevated severe storms (hail and wind).

Flash Flooding Threat:

Owing to the cool season-like synoptic setup, rapid poleward
transport of 2"+ PWATs from the 60+ kt low-level jet will "prime
the pump" for torrential to locally excessive rainfall (2"+/hour
rainfall rates). Given the wet antecedent conditions and
elevated river and stream levels across the area, it will take
less rain, even in rural areas, to become problematic. For urban
and highly developed suburban areas, the wet antecedent
conditions would only serve to exacerbate the issues even coming
out of dry patterns. In collaboration with WPC, a rare flash
flood threat level 3 of 4 (moderate risk ERO) has been issued
for Wednesday.

Repeated rounds of storms, if not true backbuilding/training,
particularly near and north of the front, meet aspects of a
Maddox archetype. With 24-hour HREF LPMM QPF pushing 3-4" in
swaths, the level 3 of 4 flood threat appears more than
reasonable. Concern would be urban flooding potentially on the
level of mid May 2020 if conditions come together just wrong
in/near Rockford, Chicago, and flashy river/stream basins in
the nearby Chicago suburbs. Expect a Flood Watch to be issued by
the overnight shift, informed by further guidance and
observational assessment.

Final Takeaway Message:

Given the severe weather parameters in play tomorrow, as well
as the threat for potentially significant flash flooding (even
outside of the higher severe threat areas), it is important to
continue to monitor later forecast updates closely. Review (and
practice) severe weather safety plans. Have multiple redundant
means to receive warning information, and keep mobile devices
charged in the event of power outages.

Castro/Izzi

Wednesday Night Onward:

We should see a break in the active pattern for the rest of the
work week. There are indications that we could get into a more
active northwest flow type pattern heading into the weekend and
beyond. This may include a threat for another strong moisture
laden system and an associated severe weather and flooding
threat on Sunday PM/Father`s Day. In other words, our break may
be short-lived, unfortunately.

Izzi/Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1233 PM CDT Tue Jun 16 2026

Main Concerns:

- Short remaining window of TS potential early this afternoon at
  Chicago metro terminals

- Gusty west-southwest to west winds until sunset this evening

- Waves of TS on Wednesday, some of which may be severe with
  torrential rainfall, especially during the afternoon

- Potential for wind variability in and near TS on Wednesday,
  favoring easterly by mid to late in the afternoon

- Low CIGs Wednesday afternoon

The cold front pushing across northern Illinois early this
afternoon will subsequently end the threat for TS. Held onto
short PROB30s at ORD, MDW, and GYY and will adjust as needed
pending any TS development (or lack thereof). Southwest winds
gusting to around 30 kt will veer to westerly behind the cold
frontal passage and then quickly ease this evening with sunset.

An unseasonably strong storm system will bring a variety of
impactful weather on Wednesday. An initial round of SHRA/TS from
mid morning through early afternoon may favor just southwest of
the TAF sites with +TS (including some severe threat), for
which PROB30s were maintained in the TAFs given uncertainty. The
time window of greatest concern is after 18z Wednesday, with a
distinct threat for strong to severe TS and +TS with 1SM (or
less) VSBY. A warm front may remain just south of the TAF sites
and keep the higher end severe weather potential south of I-80
or thereabouts. However, despite being north of the front with
east to southeast winds and CIGs deteriorating to low MVFR to at
least temporary IFR, TS north of the front may still be capable
of producing 40-50+ kt wind gusts and large hail. Confidence
was high enough for TEMPO 1SM +TSRA in the ORD and MDW TAFs.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 9 PM CDT this evening for the IL
     and IN nearshore waters.

&&

$$

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